Part of the problem seems to be that Series 3 was a huge upgrade to the series 2. Everyone was expecting a similar increase in processing power. I have never used a series 2, but given all the comments bashing TiVo about being slow and clunky, it must have been pretty bad. The series 3 responds as fast as I can push the buttons on the remote (using as a DVR not internet).
On the internet the series 3 is slow and clunky, I think people were expecting this to be fixed in the new boxes.